1. Field of the Invention
The present invention relates to a branch line monitoring system and branch line monitoring method for monitoring, for each of branch lines branched into n lines in a 1:n (xe2x89xa73) optical communications line, a loss distribution state along a longitudinal direction of the branch line including breaks thereof.
2. Related Background Art
Conventionally known as an optical line monitoring system for monitoring loss distribution states of optical fiber lines, which are optical communications lines, along their longitudinal direction including their breaks is a system utilizing an OTDR (Optical Time Domain Reflectometer). The OTDR introduces monitor light into an optical fiber line, detects as a function of time the intensity of part of the monitor light scattered backward (backscattering light) due to the loss in the optical fiber line or the like, and monitors breaks of the optical fiber line, the loss distribution state of the optical fiber line along the longitudinal direction, and the like according to the result of detection.
An example of the OTDR applied to monitoring each of a plurality of branched optical fiber lines is an apparatus disclosed in Japanese Patent Application Laid-Open No. HEI 2-141641 (hereinafter referred to as first conventional example). The loss distribution measuring apparatus in accordance with the first conventional example is provided with band filters corresponding to a plurality of optical fiber lines which are branch lines, respectively, each band filter selectively transmitting therethrough one of a plurality of channels of monitor light having wavelengths different from each other. Also, the first conventional example has such a configuration that only one channel of monitor light is propagated through its corresponding one optical fiber line, so as to sequentially monitor the respective states (including breaks and loss distribution fluctuations) of the optical fiber lines.
On the other hand, the apparatus described in IEICE SB-11-3, Spring 1997 (hereinafter referred to as second conventional example) has a configuration in which an AWG (Arrayed Waveguide Grating) is employed as a branching device for monitor light, so as to individually monitor the respective states of a plurality of branched optical fiber lines.
The inventors have studied the above-mentioned conventional techniques and, as a result, have found problems as follows.
Usually employed when monitoring a plurality of branched optical fiber lines is a configuration in which substantially the same quantity of monitor light is distributed to each optical fiber line, i.e., the intensity of each branched monitor light component introduced to the respective optical fiber line is 1/n. In such a configuration, the first conventional example monitors each of a plurality of optical fiber lines by propagating therethrough its corresponding one channel of monitor light, there by yielding a narrow dynamic range (measurement wavelength band) and low S/N ratio for each monitor light wavelength. In the second conventional example, on the other hand, operations of the AWG are sensitive to changes in temperature, thereby necessitating temperature control by use of a Peltier device or the like, which increases the cost.
Further, as described in IEICE B-588, Autumn 1991, OTDR apparatus using a light source with a narrow oscillation wavelength band for monitor light generate fading noise due to a high coherence of the light source, thereby failing to carry out favorable measurement.
For overcoming the problems mentioned above, it is an object of the present invention to provide a branch line monitoring system and branch line monitoring method having a structure which improves the S/N ratio of measured information so as to enable highly accurate measurement and can be made at a low cost.
The present invention provides a branch line monitoring system and branch line monitoring method for monitoring each of n (xe2x89xa73) branch lines into which a predetermined wavelength of signal light is introduced by way of a branching device, the branch line monitoring system and branch line monitoring method comprising a configuration for broadening the measurement wavelength band of each optical fiber line, which is an optical communications line, so as to improve the S/N ratio of measurement information, thereby enabling highly accurate line monitoring.
Namely, the branch line monitoring system realizing the branch line monitoring method in accordance with the present invention comprises: a light source for emitting n channels of monitor light having wavelengths different from the wavelength of signal light and different from each other; a monitor light introducing structure for introducing into each of the branch lines by way of the branching device the n channels of monitor light emitted from the light source; optical filters, disposed so as to correspond to the branch lines, respectively, on the corresponding branch lines or at respective end portions of the branch lines; a backscattering light detector for detecting, by way of the branching device, backscattering light components of the channels of monitor light generated in the respective branch lines; and an arithmetic unit for specifying, according to a result of detection obtained by the backscattering light detector, at least a loss distribution state of each of the branch lines along a longitudinal direction thereof (including breaks of each branch line, loss fluctuations in each branch line along the longitudinal direction thereof, their occurring sites, and the like).
In particular, each of the above-mentioned optical filters cuts off (means therefor including reflection-and absorption) respective one channel of the introduced monitor light and transmits therethrough the remaining (nxe2x88x921) channels of the signal light. When each branch line is provided with an optical filter having such a cutoff characteristic, each branch line is monitored by the (nxe2x88x921) channels of the monitor light other than the channel of monitor light cut off by the optical filter provided so as to correspond thereto, whereby each branch line can be monitored by use of a measurement wavelength band broader than that conventionally available, which improves the S/N ratio of measurement information. Namely, in the branch line monitoring system and branch line monitoring method in accordance with the present invention, the result of measurement (result of detection according to the backscattering light detector) obtained concerning each channel of monitor light includes information about (nxe2x88x921) branch lines other than the branch line provided with the optical filter for cutting it off, whereby, when one branch line subjected to monitoring is taken into consideration, it would be monitored by (Nxe2x88x921) channels of monitor light other than the cut-off channel of monitor light. Therefore, the S/N ratio of measurement information is improved as compared with the conventional systems, whereby sites of breaks, sites of loss fluctuations, and the like in each of the n branch lines can be specified with a high accuracy. Here, such a state of each branch line is obtained, after the intensity of backscattering light of each of the monitor light components generated in the branch line is detected, by way of a special arithmetic operation process by the above-mentioned arithmetic unit according to the result of detection. The above-mentioned n channels of monitor light may be emitted to the respective branch lines from the above-mentioned light source either simultaneously or sequentially at a predetermined interval of time.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.